Sensor and guide wire assemblies in which a sensor, adapted for measurements of physiological variables in a living body, such as blood pressure and temperature, is mounted at a distal portion of a guide wire are known.
For example, the U.S. Pat. No. Re. 35,648, which is assigned to the present assignee and incorporated herein by reference for the devices and techniques disclosed therein, discloses a sensor and guide wire assembly comprising a sensor element, an electronic unit, signal transmitting cables connecting the sensor element to the electronic unit, a flexible tube having the signal cables and the sensor element disposed therein, a solid metal wire, and a coil attached to the distal end of the solid wire. The sensor element comprises a pressure sensitive device, e.g. a membrane, with a piezoresistive element electrically connected in a Wheatstone bridge-type of circuit arrangement mounted thereon. Examples of Wheatstone bridge-arrangements can be found in the U.S. Pat. No. 6,343,514, which is assigned to the present assignee and the contents of which are incorporated herein by reference for devices and techniques disclosed therein.
In a sensor and guide wire assembly comprising this type of piezoresistive element, the piezoresistive element is typically made from a doped silicon sheet and exhibits sensitivity on the order of 20 mΩ/mmHg, i.e. the resistance of the piezoresistive element changes by 20 mΩ when the ambient pressure changes by 1 mmHg. To be able to reliably detect such a small change in resistance, it is consequently required that all other resistances, both the ones that are mounted in the Wheatstone bridge and the ones that accidentally can appear between different components and between components and surrounding matter (e.g. blood), are controlled within less than 20 mΩ. In other words, the rather low sensitivity may potentially affect the reliability of the pressure measurements made by this type of blood pressure sensor, and therefore imparts very high requirements on the production processes of the corresponding sensor and guide wire assembly, something which adds to the production costs. The rather low sensitivity of the conventional sensor element also requires that the dimensions of the membrane, on which the sensor element is mounted, are relatively large, because a large membrane will deflect more than a small membrane, and a large membrane will therefore subject a sensor element mounted thereon to more stress. For various reasons there is, however, an ever ongoing struggle to reduce the dimensions of a sensor guide wire, and the conventional sensor elements therefore present a hindrance in these efforts.
Consequently, there is still a need for an improved guide wire mounted pressure sensor, by which pressure measurements can be performed with higher sensitivity. Another need is to provide a sensor element with higher sensitivity such that a sensor and guide wire assembly with smaller dimensions can be produced.